This invention relates generally to plasma etch systems used to process integrated circuit wafers and more particularly to methods and apparatus for detecting endpoint in plasma etch systems.
Plasma etch systems etch layers of an integrated circuit wafer by causing positively charged ions to bombard the surface of the wafer. The ions are produced within a plasma discharge over the wafer and are accelerated towards the wafer by a negatively charged cathode. The plasma discharge within standard plasma etch systems is created by applying radio-frequency (RF) power to both the cathode and an anode of the etch system, while the plasma discharge within reactive ion etch (RIE) systems is created by applying RF power to the cathode alone. In either system, the etch process is highly anisotropic due to the substantially perpendicular acceleration of the positive ions towards the plane of the wafer. Anisotropic etching is desirable because it permits very narrow linewidths to be formed in the integrated circuit wafer.
It is very important to determine when the endpoint of the etch process has been reached. Prior to endpoint some of the etched layer remains and, therefore, the process is incomplete. After endpoint, excessive over-etching can occur, possibly damaging lower layers of the integrated circuit wafer. While a small amount of over-etching is common to ensure complete etching of the desired layer, it is still important to know when nominal endpoint has been reached to achieve accurate and repeatable etching.
There are a number of prior art methods for monitoring endpoint. One such method uses laser interferometry to monitor the layer as it is being etched. Another method monitors the impedance or D.C. bias of the cathode. A third method, and the one most closely related to the present invention, optically monitors the emission of the plasma discharge to determine endpoint conditions.
An example of a method for optically monitoring the plasma discharge within a plasma etch chamber is taught in U.S. Pat. No. 4,312,732 of Degenkolb et al. who describe the optical monitoring of a portion of a spectrum developed by a plasma discharge for changes which would signal endpoint. More particularly, Degenkolb et al. monitor the radiation from a preselected excited species including particles resulting from the chemical combination of entities from the wafer with entities from etchant gasses within the plasma discharge. When the radiation associated with the preselected species diminishes below a predetermined threshold value Degenkolb et al. consider endpoint to have occurred.
A disadvantage of monitoring a single portion of a spectrum as taught by Degenkolb et al. is that the signal-to-noise ratio (SNR) of the signal can be quite low for certain processes. This can cause false readings of endpoint, possibly causing damage to the integrated circuit wafer being processed. Another problem with monitoring a single spectral portion is that endpoint detection is optimized for only one etch process. For example, if different etchant gas concentrations were used or if different materials were being etched the spectral portion chosen for the original etch process might be inappropriate for the new etch process. Furthermore, the prior art method of monitoring a single spectral portion is not well adapted to multiple-step etch processes where a wafer is sequentially subjected to a variety of processing conditions. Multi-step processes can be used to etch a single layer under varying conditions, or can be used to etch multiple layers of differing types.